This invention relates to liquid-crystal display device and, more particularly, to an active liquid-crystal display device having a large display area and capable of display with a high resolution.
FIG. 1 shows a prior art liquid-crystal display device. The device comprises opposed transparent substrates 11 and 12 of glass or the like which are spaced apart by a spacer 13 provided along their edges and constitute a liquid-crystal cell 10 with a liquid crystal 14 sealed between them. A plurality of display electrodes 15 are formed on the inner surface of the substrate 11. Also, a thin film transistor 16 is formed as a switching element adjacent to each display electrode 15 with the drain connected thereto. A transparent common electrode 17 is formed on the inner surface of the other substrate 12 such that it opposes the display electrodes 15.
Each of the display electrodes 15 constitutes a picture element, that is, a pixel, for instance. As shown in FIG. 2, the display electrodes 15 are square in shape and are arrayed in a matrix on the transparent substrate 11. Gate bus lines 18 are each formed in the vicinity of and along each row of display electrodes 15 in the matrix. Source bus lines 19 are each formed in the vicinity of and along each column of display electrodes 15 in the matrix. Thin-film transistors 16 are formed at the intersections of the gate and source bus lines 18 and 19. Each thin-film transistor 16 has its gate connected to the associated gate bus line 18, its source connected to the associated source bus line 19, and its drain connected to the corresponding display electrode 15.
When a voltage is applied between a selected one of the gate bus lines 18 and a selected one of the source bus lines 19, the corresponding thin-film transistor 16 is turned on, whereby the corresponding display electrode 15 is charged through the thin film-transistor 16. As a result a voltage is applied across only a portion of the liquid crystal 14 that is found between the corresponding display electrode 15 noted above and common electrode 17, whereby only an area of that display electrode 15 is rendered transparent or light-blocking. In this way, a selective display of display electrodes 15 is obtained. The display can be erased by causing discharge of the display electrode 15.
In the prior art, the thin film transistor 16 is constructed as shown in FIGS. 3 and 4. As is shown, display electrodes 15 and source bus lines 19 of a transparent conductive film of ITO or the like are formed on the transparent substrate 11. An amorphous-silicon (a-silicon) or like semiconductor layer 21 is formed such that it strides the gap between opposite edges of the display electrode 15 and an associated source bus line 19. A gate insulating film 22 of silicon nitride or the like is formed on the semiconductor layer 21. A gate electrode 23 is formed on the gate insulating film 22 above the semiconductor layer 21 such that the gate electrode 23 partly overlies the display electrode 15 and source bus line 19. The gate electrode 23 has one end connected to the gate bus line 18. Portions of the display electrode 15 and source bus line 19 facing the gate electrode 23 constitute drain and source electrodes 15a and 19a, respectively. The thin-film transistor 16 is constituted by the drain and source electrodes 15a and 19a, semiconductor layer 21, gate insulating film 22 and gate electrode 23. The gate electrodes 23 and gate bus lines 18 are formed simultaneously from aluminum, for instance. Ohmic contact layers 25 and 26 of n.sup.+ type semiconductor, for instance, are formed between the semiconductor layer 21 and the respective electrodes 15a and 19a to reduce adverse effects on the operation characteristics of the thin film transistor 16.
If a liquid-display device of the type described above is to have a large display area and a high resolution of display, a large number of display electrodes 15 should be formed at a high density. Also, the source bus lines 19 should be of a considerable length. This means that because of a voltage drop the potential at a point on each source bus line 19 becomes lower as the point departs from one end of the source bus line connected to the voltage supply. In other words, a brightness gradient is produced on the display such that the brightness of a pixel becomes lower as the pixel is located farther away from the end connected to the voltage supply terminal.
In another aspect, the amorphous silicon constituting the semiconductor layer 22 has photo-conductivity. Therefore, when the semiconductor layer 22 is illuminated by external light, the thin-film transistor can not provide a sufficiently high "off" resistance (i.e., resistance between the source and drain) even when the transistor is in an off state. Further, to form the electrodes and bus lines at high densities, the etching for the formation of the pattern of electrodes 15 and source bus lines 19 is usually effected by an anisotropic dry etching process in a direction perpendicular to the plane of the substrate 11. Therefore, if the thickness of the source bus lines 19 is increased to reduce the resistance thereof, a continuous semiconductor layer could not be formed at the edges of the drain and source electrodes 15a, 19a; in other words, a channel portion of the semiconductor layer 21 between the drain and source electrodes would be separated from the semiconductor layer portions riding on the drain and the source electrodes 15a, 19a. This means that the thickness of the semiconductor layer 22 can not be greatly reduced, and at least a thickness of about 1,000 angstroms is necessary.
Further, the ohmic contact layers 25 and 26, which are formed on the side of the electrodes 15a and 19a opposite the substrate 11, has a small width. Therefore, it is difficult to obtain satisfactory ohmic contact of the electrodes 15a and 19a with the semiconductor layer 22. For this reason, an off-set is produced in the drain current versus drain voltage characteristic.
Further, if the electrostatic capacitance between the source or gate bus line 19 or 18 and common electrode 17 is large, it is difficult to obtain a high speed operation. Besides, since the portion of liquid crystal between the source or gate bus line 19 or 18 and common electrode 17, which portion is not desired to contribute to any display, is also applied with a DC voltage, the possibility of deterioration of the liquid crystal would increase as the area of this liquid crystal portion increases.